1. Field of the Invention
The present invention relates to alumina abrasive grains and more particularly to abrasive grains used for grinding operations.
2. Brief Description of the Prior Art
There are a variety of grinding operations under loads ranging from heavy grinding to extremely heavy grinding. For instance, stock removal and conditioning of steel ingot or slab, deburring or cutting-off of burs and runners of steel casting or the like are heavy grinding operations. Usually special abrasive grains are employed for the aforesaid heavy grinding operations.
Hitherto, typical abrasive grains for the heavy grinding operations as described above are those that are manufactured by sintering bauxite grain (c.f. Japanese Patent Publn. No. 4398/64, No. 27612/64 etc.). It is pointed out that as drawbacks of the hitherto known abrasive grains, that they have a reduced hardness due to inclusion of large quantities of foreign material other than alumina, such as silica, iron oxide and the like and that they may be employed only in a limited scope of application such as for the grinding of metallic materials composed mainly of stainless steel. This is because of the existence of a solid solution structure such as a glass-like structure or the like associated with these grains, causing them to be undesirable for the grinding of high hardness materials such as cast iron, chrome steel, spring steel and the like.
Usually high hardness metal materials such as special steel and the like are ground under a heavy pressure. In order to carry out such extremely heavy grinding operations, abrasive grains are available, which are manufactured by mixing alumina and zirconia powder, electrofusing them and then quenching them. However, this type of abrasive grains also has drawbacks in that the grains are very costly to manufacture. Generally speaking, the process of manufacturing fused type abrasive grains involves additional steps of crushing and sieving after completion of solidification of the molten material, during which a variety of grains sizes of abrasive grains are generated. It would be preferable, if all abrasive grains ranging from fine to coarse sizes could be used in many fields of application. However, the alumina-zirconia abrasive grains of coarse size only, are useful for heavy or extremely heavy load operations. Abrasive grains having grains sizes finer than that useful for the aforesiad heavy or extremely heavy load (pressure) operations cannot be used and are subjected to remelting. The problem is that the unusable portion occupies a considerably high percentage of the whole of the manufactured abrasive grains, which causes an increased manufacturing cost. Moreover, there is another problem with the latter composite abrasive grains, because of anticipated difficulty in securing a supply source of high purity zirconia. Accordingly it is expected that alumina-zirconia abrasive grains will become even more expensive. In view of the aforesaid problems with the currently available abrasive grains, development of other, replacement abrasive grains, has been looked for.
It goes without saying that good abrasive grains are identified by "sharp cutting with less wearing". To ensure the high performance capability of abrasive grains, it is necessary that the hardness of the abrasive grains be relatively higher than that of the workpiece to be ground. Under that condition, wearing of the abrasive grains is substantially reduced. Further and particularly in the case of grinding operations under heavy pressures, the abrasive grains require a high bending strength and compressive strength under the heavy load. Also, sharp cutting not only requires that the abrasive grains have high hardness and strength but also that they have a capability of generating a cutting edge so that new cutting edges are successively provided during the grinding operation. This is one of the most important properties for the abrasive grains. To ensure the generation of new cutting edges, it is necessary that chipping take place on the abrasive grains, at the extreme end portion thereof in contact with the workpiece. Chipping should occur to the most effective extent, in respect to quantity of material removal and the time interval or frequency of chipping. Of course this chipping occurs in a different manner, depending on a number of operating conditions such as applicable load etc. If no chipping takes place during the grinding operation, hot sticking or the like occurs on the workpiece because no new cutting edge is generated after the old cutting edge has been worn away. This eventually leads to stoppage of the grinding operation. On the other hand, excessive chipping is not desirable, because the abrasive grains wear too rapidly. Hence it is most preferable that chipping occur at the extreme end of the abrasive grain, to the minimum extent required for generation of new cutting edges at adequate time intervals during the grinding operation.
We have found that the fundamental physical properties of sintered abrasive grains depend on the density thereof, crystal particle size, and distribution of the crystal particles. The density is related to mechanical strength of the abrasive grain. As density increases the strength and hardness of the grain are correspondingly increased. Further the particle size of the crystal has an effect on the mechanical strength as well as on the extent of wearing of the abrasive grain. Abrasive grains having a small and compact crystal structure exhibit increased strength and resistance against wearing. In view of these facts, it is important to inhibit the growth of crystal particles and increase density of the grains during sintering.
The distribution of crystal particles in the abrasive grain is also related to the above described generation of a cutting edge, which is one of the important properties of the abrasive grains.
For instance, there are various kinds of sintered alumina abrasive grains which are manufactured by molding fine alumina powder and sintering the molded material at a variety of temperatures. In order to increase the density after completion of sintering, very finely crushed alumina is preferably used and further a small amount of inhibitor such as Cr.sub.2 O.sub.3, MgO, NiO or the like is added thereto so as to inhibit crystal growth, without any substantial increase in the sintering temperature. It is recognized as a drawback with this type of sintered alumina abrasive grains, that they may be employed only in a limited scope of applications because of their possessing less capability of generating of cutting edges due to their inherent structure. This is in spite of the fact that they are wholly composed of a uniform and fine sintered structure and meet satisfactorily the requirements for density and hardness after the completion of sintering. In order that the generation of cutting edges takes place with such abrasive grains, grinding under excessively heavy load conditions is necessary. Without excessive pressure (300 to 1,000 Kg of grinding wheel pressing load) chipping doesn't take place with this type of sintered alumina abrasive grain. This necessitates that the grinding wheel made of the sintered alumina abrasive grains be subjected to frequent dressing with the aid of a diamond tool or the like during the grinding operation. This results in decreased efficiency and increased wearing of the grinding wheel.
Moreover, among the known sintered alumina abrasive grains, there are those which are manufactured by the steps of mixing electrofused fine alumina powder with fine bauxite powder, molding the well mixed material and sintering the molded material. It is pointed out that with this latter type of abrasive grain, they have no problem in generating a cutting edge. However, they have a drawback in that the sintered bauxite portion located between the electrofused alumina grains has a low hardness, causing increased wear of the abrasive grain.
Working conditions in grinding operations under heavy loads are different, respectively, depending on the type and shape of workpiece to be ground, purpose of the grinding operation and grinding machine characteristics. Since there is a definite distinction between heavy grinding and extremely heavy grinding operations, it is necessary to provide abrasive grains which satisfactorily meet the requirements for the respective working conditions.